George Osborne will pledge to introduce even deeper spending cuts after the next general election by imposing a new cap on parts of the welfare budget and other areas which have so far been immune from the coalition's austerity plans.

As the chancellor served notice that Britain faces a longer-than-expected recovery, after the Office for Budget Responsibility halved its growth forecast for this year, a senior Treasury source said Osborne would promise to target so called "annually managed expenditure" (AME) in the next parliament.

This accounts for about 50% of government spending – covering many areas of welfare, debt interest and EU contributions – and has largely not been targeted during the current round of spending cuts. The chancellor has instead focused the cuts on the other area of government spending – the so-called departmental expenditure limits (DEL) which cover the day-to-day running of Whitehall.

The announcement by the Treasury in the traditional post-budget briefing came after the chancellor said he had delivered a "budget for our aspiration nation" that would boost economic growth and help families struggling with the cost of living.

Osborne, who struggled with a sore throat during his 55-minute statement, delivered bad economic news – the growth forecast for 2013 has been halved from 1.2% to 0.6% – but some surprises for what he called "hard-working families" and employers. He said he would:

• Offer guarantees to support £130bn worth of mortgages for three years from 2014. It is expected that this will help 75,000 homeowners. The government will introduce a £3.5bn scheme to offer a five-year interest-free loan worth up to 20% of the value of a new-build home worth no more than £600,000. This will help 190,000 people a year.

• Introduce a new employment allowance which will mean employers will not have to pay the first £2,000 of employer national insurance contributions.

• Scrap the beer duty escalator and cut beer duty by 1p. He also announced he would scrap an increase in fuel duty, due in September.

A Treasury source said the chancellor would outline a "forward-looking limit on AME spending" in his spending review on 26 June that is due to cover 2015-16. The source said: "The UK stands out internationally because only half of public spending is subject to tight controls. The IMF and others think that is quite a low percentage for an advanced economy. Others have much higher percentages.

"So we are going to set out how we are going to set out an overall cap on that AME spending. That will evolve in the future once that is up and running. If AME spending is running ahead of forecasts, tough decisions on things like welfare [will have to be made] in order to stay within that cap. So it is part of applying more consistent ongoing controls for welfare and the rest of the AME budget."

Osborne rounded on Labour as he confirmed that the government would raise the tax-free personal allowance – the first Liberal Democrat manifesto pledge – to £10,000 next year, a year earlier than originally planned.

Ed Miliband condemned Osborne as a "downgraded chancellor" as he mocked him on the day that he launched an official Twitter account. "All he has to offer is just a more-of-the-same budget. Today the chancellor joined Twitter. He could have got it all into 140 characters: Growth down, borrowing up, families hit, and millionaires laughing all the way to the bank. Hashtag downgraded chancellor."

Osborne made clear that his sights remained firmly fixed on the spending round in June when the Treasury announces the new focus on welfare spending. The Treasury source said the IMF and other international bodies recommended that 70-80% of government spending should be subject to "strict controls". Britain only does this on 50% of spending.

The source said the "automatic fiscal stabilisers", which make up the gap between falling tax receipts and rising welfare bills during a downturn, would remain in place. The changes would target "the structural growth in welfare spending" such as the increase in incapacity benefit in recent decades.

The source said: "What has happened in the past is that particular benefits have turned out to be more expensive than initially thought. Something went structurally wrong with incapacity benefit.

"Eventually we are dealing with incapacity benefit and we have replaced it with employment support grants. That took a very long time to happen. One of the things this system might do is pick up that structural increase and say this is a problem, you are going to breach your spending cap and force governments to take earlier action to deal with structural changes in things like welfare spending."

The announcement came as the OBR confirmed that it has halved its growth forecast for 2013 from 1.2%, forecast last December, to 0.6%. But it has revised upwards its forecast for growth last year.

The chancellor hailed the government's success in tackling the fiscal deficit, which he has now cut by one third. Robert Chote, the chairman of the OBR, said the deficit had fallen from £160bn in 2009-10 to £120bn in 2011-12 as a result of the spending cuts. But Chote said he expected the deficit to remain at around £120bn until it starts dropping in 2014-15 onwards. The deficit will fall as a share of national income but more slowly than expected, he said.

Chote said the chancellor was on course to meet his fiscal mandate for the cyclically adjusted current budget in balance or surplus five years ahead which, in this budget, means 2017-18. "Our central forecast shows the cyclically adjusted current budget in surplus by 0.8% of GDP in 2017-18 which means that we think the government does have a better than 50% chance of meeting the mandate on current policy," he said. Osborne had originally suggested he would meet this target by the time of the 2015 election.

But Chote said Osborne would miss his "supplementary target" of seeing debt falling as a share of GDP by 2015-16. "We now expect it [public sector net debt] to rise for a further year [from 2015-16] peaking at 85.6% of GDP in 2016-17 rather than at 79.9% in 2015-16 as we said in December."

The chancellor suffered a blow minutes before he stood up when the London Evening Standard tweeted a picture of its front page which contained key details of the budget. A member of staff was suspended for tweeting the front page in breach of a Treasury embargo.